1. Field of the Invention
The present invention relates to a plug structure for sealing a hole which is formed on a housing or the like of a fluid device sealed with fluid therein.
2. Discussion of the Related Art
For example, fluid devices used in the field of this related art include a brake system with a hydraulic circuit for a motor car and the like and also include a plunger pump used as a hydraulic pressure source for an accessory system such as antiskid system, power steering system or the like. In a plunger pump described in JP9-209941 A, there is shown a structure that a sealing plug is press-fitted in a port formed on a housing and that the circumferential edge of an opening end portion of the port is caulked to prevent the sealing plug from coming off due to the pressure acting in a fluid passage as well as to fluid-tightly seal the port.
Further, JP11-280982 A shows another structure that a steel ball is press-fitted in a port formed on an aluminum housing and that the circumferential edge of an opening end portion of the port is caulked to prevent the steel ball from coming off due to the pressure acting in the port as well as to fluid-tightly seal the port.
As a method of forming a sealing structure by press-fitting a steel ball in a port and by caulking the circumferential edge of an opening end portion of the port in the manner described above, the following technology has heretofore been known for example. First of all, as shown in FIG. 7, a steel ball 80 is press-fitted by a first punching member 82 from the outside in an end portion of a port 85 of a housing 84 to be pushed against a stop holding portion 86 at an inner side. Then, as shown in FIG. 8, the circumferential edge 90 of the opening end portion is struck by a second punching member 88 from the outside to caulk the circumferential edge 90 of the opening end portion. Further, as shown in FIG. 9, the circumferential edge 90 of the opening end portion is repetitively struck by the second punching member 88 to make the opening end portion narrow. Finally, as shown in FIG. 10, the striking is further repeated to bring the circumferential edge 90 of the opening end portion into contact with the outside end of the steel ball 80. As shown in FIG. 11, this results in covering the outside of the steel ball 80 mainly through the plastic deformation of the circumferential edge 90 of the opening end portion to hold the steel ball 80 at the plastically deformed circumferential edge 90 of the opening end portion as well as at the stop holding portion 86. In use, a hydraulic pressure acts on the steel ball 80 in the direction indicated by the arrow shown at a lower part of the figure.
However, in the sealing structures described in the aforementioned Japanese applications, only the circumferential edge of the opening end portion deformed by caulking serves to bear a high pressure exerted on the sealing plug or the steel ball. Thus, it is difficult to secure the strength of the circumferential edge which is deformed by caulking at the opening end portion of the port for preventing the sealing plug or the steel ball from coming off the port, and therefore, a problem may arise in that it is insufficient to prevent the sealing plug or the steel ball from coming off due to the high hydraulic pressure acting in the fluid passage. In addition, in the steps of forming the sealing structure, the caulking has to be done by using two kinds of punching members including the first and second punching members, and thus, there arise another problem that an increase in the manufacturing steps causes the manufacturing cost to rise.